Electrical appliances for use in a railroad locomotive often derive operating power from a battery through power distribution lines. The power distribution lines, in this case the battery circuit, may deliver high surge voltages to an appliance's power input. International rail electronics standards require that electrical appliances used in locomotives should be able to withstand applications of energetic, high voltage surge pulses and transients carried through the power distribution lines. In addition, non-rail electronics may have similar requirements to withstand surge and transient pulses. However, providing a power supply for appliances having a fixed voltage input, that operates normally within a wide range of voltages, and survives high energy surges and transients is technically challenging. Additionally, limiting inrush current into the electrical appliances upon initial application of the power provides additional challenges. Furthermore, appliances must limit emissions of high frequency currents into the power distribution lines. This requirement typically bears the moniker, “conducted emissions”. Successful conformance to this standardized requirement prevents consequent emission of RF interference from the power distribution lines.
European Patent Number 1,720,239 (the '239 patent) provides a DC/DC converter that comprises a switch-mode power supply with a switching transistor, in particular a MOSFET transistor, which is coupled between a DC voltage supply and a load, and a controlling circuit with a controller, which delivers driving pulses for the switching transistor, and a transformer between the controller and the switching transistor. The controlling circuit further comprises a primary circuit with a capacitor and a clamping circuit with a capacitor. A main feature of the '239 patent is that the controlling circuit further comprises a timing circuit, which switches the switching transistor off after a certain on-time of the controller. However, the '239 patent does not provide a power supply for protecting electrical appliances from an exposure to a wide voltage range, high voltage surge transients, and the generation of conducted emissions without suffering unduly large thermal loads.